Without limiting the scope of the invention, its background is described in connection with the diffusion of dopants in the base layer of a heterojunction bipolar transistor (HBT), as an example.
Heretofore, in this field, it has been the object of device designers to develop HBTs to provide signal gain at microwave frequencies. A necessary consequence of this design goal is the formation of a structure that exhibits a low series base resistance. The speed of HBTs depends strongly on the device base resistance, which must be kept low to minimize resistive parasitics of the input.
Self-aligned emitter-base contact fabrication techniques are often used to minimize the series base resistance by lowering the distance between these two contacts. The lowering of the base sheet resistance is, however, the most effective way to achieve a reduction in all components of the base resistance (series resistance, contact resistance, and access resistance).
Low base sheet resistance in npn HBTs with base layers made from compound semiconductor materials, such as GaAs, has been achieved by heavily doping the base layer with Be or Zn. Base doping concentration in excess of 1.times.10.sup.19 cm.sup.-3 is needed for microwave performance. Such heavily doped base layers, however, are not stable under high temperature growth or under high temperature and bias operating conditions. Degenerately doped base layers can act as diffusion sources under these type of stress conditions and p-type dopant can diffuse into the emitter and/or the collector layer irreversibly changing the device parameters. The presence of a heavily doped n-type layer close to the p-type doped layer also causes diffusion enhancement during layer growth at elevated temperatures (typical growth temperatures in MOCVD is 600.degree.-800.degree. C.).